Circuit and method for reducing stored energy in an electrosurgical generator

ABSTRACT

A circuit for discharging stored energy in an electrosurgical generator includes a pulse width modulator for controlling a high voltage power supply, an error signal generating circuit configured for delivering an error signal as a difference between an output signal voltage and a feedback voltage generated by the high voltage power supply. The circuit further includes a switching circuit configured to switch in a load in parallel with an output of the high voltage power supply when the error signal is less than a first predetermined threshold to discharge the output.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. application Ser. No.11/256,374, filed on Oct. 21, 2005, now U.S. Pat. No. 8,734,438, theentire contents of which are hereby incorporated by reference herein.

BACKGROUND

1. Field

The present disclosure relates generally to electrosurgical system, andmore specifically, to a system and method for discharging excess energyof a high voltage direct current (HVDC) power supply of anelectrosurgical generator.

2. Description of the Related Art

Electrosurgery involves application of high radio frequency electricalcurrent to a surgical site to cut, seal, ablate, or coagulate tissue. Inmonopolar electrosurgery, a source or active electrode delivers radiofrequency energy from the electrosurgical generator to the tissue and areturn electrode carries the current back to the generator. In monopolarelectrosurgery, the source electrode is typically part of a surgicalinstrument held by the surgeon and applied to the tissue to be treated.A patient return electrode is placed remotely from the active electrodeto carry the current back to the generator.

In bipolar electrosurgery, a hand-held instrument typically carries twoelectrodes, e.g., electrosurgical forceps. One of the electrodes of thehand-held instrument functions as the active electrode and the other asthe return electrode. The return electrode is placed in close proximityto the active (i.e., current supplying) electrode such that anelectrical circuit is formed between the two electrodes. In this manner,the applied electrical current is limited to the body tissue positionedbetween the two electrodes.

Conventional electrosurgical generators include a high voltage directcurrent (HVDC) power connected to a radio frequency (RF) output stage,which converts DC energy generated by the HVDC into RF energy. The highvoltage direct current (HVDC) power supply includes an output filterwhich smoothes the switching of the HVDC into a DC level. This filtercan store large amount of energy and under light loads and highimpedance, the discharge of the output filter is slow. As a result, thegenerator response time is significantly lowered thereby limiting thegenerator's ability to pulse energy rapidly or respond quickly duringlight loads.

Therefore, there is a need for an electrosurgical generator which candischarge energy in a consistent and rapid manner under various loadconditions, including light loads and high impedance.

SUMMARY

The present disclosure provides for an electrosurgical generator whichincludes a circuit for discharging stored energy and a high voltagepower supply. The active discharge circuit includes a pulse widthmodulator, a load having a resistive element and a switching circuit,and an error signal generating circuit. The error generating circuitdetermines a difference between and output set point voltage andfeedback voltage and generates an error signal. If the error signal isless than a first predetermined threshold the switching circuit switchesin a load and sinks current supplied by the high voltage power supplythrough the load. If the signal is above a second predeterminedthreshold the pulse width modulator is switched on. This ensures thatthe pulse width modulator and the load are not active simultaneously.

According to one embodiment of the present disclosure a circuit fordischarging stored energy in an electrosurgical generator is disclosed.The circuit includes a pulse width modulator for controlling a highvoltage power supply, an error signal generating circuit configured fordelivering an error signal as a difference between an output signalvoltage with a feedback voltage generated by the high voltage powersupply. The error signal is transmitted to the pulse width modulatorwhen the error signal is large enough the pulse width modulator turnson. The circuit further includes a switching circuit configured toswitch in a load in parallel with an output of the high voltage powersupply when the error signal is lesser than a first predeterminedthreshold to discharge the output.

According to another embodiment of the present disclosure anelectrosurgical generator is disclosed. The generator includes a highvoltage power source for generating direct current, a radio frequencyoutput stage for converting direct current into radio frequency energy,and a circuit for discharging stored energy. The circuit includes apulse width modulator for controlling a high voltage power supply, anerror signal generating circuit configured for delivering an errorsignal as a difference between an output signal voltage with a feedbackvoltage generated by the high voltage power supply. The error signal istransmitted to the pulse width modulator. The circuit further includes aswitching circuit configured to switch in a load in parallel with anoutput of the high voltage power supply when the error signal is lesserthan a first predetermined threshold to discharge the output.

According to a further aspect of the present disclosure a method fordischarging energy stored in a circuit in an electrosurgical generatoris disclosed. The method comprises the steps of deriving an error signalas a difference between an output setpoint voltage with a feedbackvoltage generated by a high voltage power supply, comparing the errorsignal with a first predetermined threshold, switching on a load inparallel with an output of the high voltage power supply when the errorsignal is lesser than a first predetermined threshold to discharge theoutput, and switching on a pulse width modulator if the error signal isabove a second predetermined threshold.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features, and advantages of the presentdisclosure will become more apparent in light of the following detaileddescription when taken in conjunction with the accompanying drawings inwhich:

FIG. 1 is a schematic block diagram of one embodiment of anelectrosurgical system according to the present disclosure;

FIG. 2 is a schematic block diagram of a generator according to thepresent disclosure;

FIG. 3 is a schematic block diagram of a high voltage direct current(HVDC) power supply according to the present disclosure;

FIG. 4 is a circuit diagram of the HVDC power supply according to thepresent disclosure; and

FIGS. 5A-5F are graphs of the HVDC response with sinusoidal input andoutput waveforms generated by the HVDC power supply of the presentdisclosure.

DETAILED DESCRIPTION

Preferred embodiments of the present disclosure will be described hereinbelow with reference to the accompanying drawings. In the followingdescription, well-known functions or constructions are not described indetail to avoid obscuring the present disclosure in unnecessary detail.

The present disclosure provide for an electrosurgical generatorincluding a high voltage power supply for supplying a DC voltage, anoutput filter, and an active discharge circuit for effectivelydischarging stored energy in the output filter. The active dischargecircuit switches in a load in parallel with the output filter so thatenergy stored in the output filter is discharged in consistent mannerregardless of the resistance of the external load.

The generator according to the present disclosure can be used withbipolar and monopolar electrosurgical devices. FIG. 1 is a schematicillustration of a monopolar electrosurgical system 1. The system 1includes an active electrode 14 and a return electrode 16 for treatingtissue of a patient P. Electrosurgical RF energy is supplied to theactive electrode 14 by a generator 10 via a cable 18 allowing the activeelectrode 14 to ablate, cut or coagulate the tissue. The returnelectrode 16 is placed at the patient P to return the energy from thepatient P to the generator 10 via a cable 15.

The generator 10 may include input controls (e.g., buttons, activators,switches, etc.) for controlling the generator 10. The controls allow thesurgeon to adjust power of the RF energy, waveform, and other parametersto achieve the desired waveform suitable for a particular task (e.g.,cutting, coagulating, etc.). Disposed between the generator 10 and theactive electrode 14 on the cable 18 is a hand piece 12, which includes aplurality of input controls which may be redundant with certain inputcontrols of the generator 10. Placing the input controls at the handpiece 12 allows for easier and faster modification of RF energyparameters during the surgical procedure without having the surgeondivert his attention from the surgical site and returning to thegenerator 10.

FIG. 2 shows a schematic block diagram of the generator 10 having acontroller 26, a high voltage DC power supply (HVDC) 28, and an RFoutput stage 30. The controller 26 includes a microprocessor and anoutput port of the microprocessor is electrically connected to the HVDC28. The HVDC 28 is configured to supply DC power to the RF output stage30. The controller 26 receives input signals from the generator 10and/or hand piece 12, e.g., a set point, and the controller 26 in turnadjust power outputted by the generator 10, more specifically the HVDC28, and/or performs other control functions thereon.

The RF output stage 30 converts DC power into RF energy and delivers theRF energy to the active electrode 14. In addition, the RF output stage30 also receives RF energy from the return electrode 16. The power ofthe HVDC 28 can be varied to modify RF magnitude (e.g., amplitude)thereby adjusting the power of the RF energy delivered to the tissue.This allows for accurate regulation of the power of delivered RF energy.

Regulation of output energy is controlled by the controller 26 (e.g., amicroprocessor) using algorithms and/or software. The controller 26forms a closed-control loop with a sensor 29 which senses various tissueand output energy properties and reports the properties data to thecontroller 26. The closed-control loop allows for real-time adjustmentof output energy based on the properties sensed by the sensor 29. Morespecifically, the closed-control loop can process signals from thesensor 29 and make corresponding adjustments in about 250 μs. The HVDC28 is capable of supplying and discharging the current at similar rates(e.g., sourcing at about 300 V/ms or faster and discharging at about 7V/ms or faster).

With reference to FIG. 3, discharging of current is accomplished usingan active discharge circuit (ADC) 31—a component of the HVDC 28—which isa circuit that switches a load 36 (e.g., one or more resistors) inparallel with an output capacitor 34. The HVDC 28 includes a diode 33 onan output connection 35 which allows current to flow away from the HVDC28 into a filter 32. The load 36 discharges the energy stored in thecapacitor 34. During discharge, the diode 33 prevents the dischargedcurrent to flow back into the HVDC 28 thereby directing the currenttoward the active electrode 14.

FIG. 4 shows the ADC 31 in more detail and other components of thegenerator 10. The output and input connections 35, 37 include aplurality of diodes 33 which block the output current from returning tothe HVDC 28 during discharge. The HVDC 28 is connected to a pulse widthmodulator 38, which may be a Pulse Width Modulator UCC3895 availablefrom Texas Instruments, for controlling the output of the HVDC 28. Thepulse width modulator 38 implements control of a full-bridge power stage33 by phase shifting the switching of one half-bridge with respect tothe other. It allows constant frequency pulse-width modulation inconjunction with resonant zero-voltage switching to provide highefficiency at high frequencies and can be used either as a voltage modeor current mode controller.

The pulse width modulator 38 is configured to receive an error signalwhich is generated by an error signal generating circuit, a firstcomparator 52. The error signal is derived from the difference betweenthe output set point of the HVDC 28 (e.g., ECON)—the intended outputvoltage—and the voltage feedback of the HVDC 28 (VFB)—actual outputvoltage generated by the HVDC 28. If VFB is higher than ECON, withoutthe ADC 31, the signal would be 0V. This causes the loop to be delayedas it waits for the signal to increase in order to activate the pulsewidth modulator 38. The ADC 31 avoids that problem and maintains thesignal from dropping too low because the ADC 31 discharges the output ofthe HVDC 28 faster than the signal may drop. The error signal is sent tothe shift controller 38 which compares the error signal with a secondpredetermined signal. If the signal drops below the second predeterminedthreshold, about 0.7V, the pulse width modulator 38 shuts down and theADC 31 becomes operational. When the pulse width modulator 38 shutsdown, the HVDC 28 stops sourcing current.

The signal is also compared against a first predetermined threshold,about 0.5V, at a second comparator 50. Thus, if the signal is below thislevel the ADC 31 will turn on. This ensures that the pulse widthmodulator 38 will not be turned on when the ADC 31 is on therebyreducing chance of HVDC 28 driving into the ADC 31. The signal feedinginto the second comparator 50 is not filtered, this allows for arelatively fast response from the ADC 31. The time period between pulsewidth modulator 38 shutting down and the ADC 31 starting up, or viceversa, is about 5 μs.

As discussed above, the capacitor 34 is in parallel with the load 36which is used to discharge the current. The load 36 provides a gatedrive voltage and includes a switching component 40 and a resistiveelement 42. The switching component 40 can be a transistor, such as afield-effect transistor (FET), metal-oxide semiconductor field-effecttransistor (MOSFET), insulated gate bipolar transistor (IGBT), relay,and the like. The resistive element 42 is in series with the switchingcomponent 40 and to ground 44, which is known as a source follower. Thesource follower limits amount of current which flows through theresistive element 42. As amount of current flowing through the resistiveelement 42 increases, the voltage across the resistive element 42increases as well. This voltage subtracts from the gate drive voltage asthe current reaches a predetermined threshold causing the switchingcomponent 40 to turn off thereby acting as a variable resistor. Theresistive element 42 has a resistance, such as about 5 Ohms, which willlimit the current to less than about 2 Amps. For instance, the resistiveelement 42 has a turn on around 2V and 2 A and will subtract 10B (2 A*5Ohms) from the 12V gate drive. This reduces the stress on HVDC 28 andother output components.

EXAMPLES

FIGS. 5A-5E are graphs of HVDC 28 output. FIG. 5A shows output responseof the HVDC 28 without the ADC 31 while FIG. 5B shows output response ofthe HVDC 28 equipped with the ADC 31. Waveform 60 represents ECONvoltage (e.g., the intended output voltage) that fluctuates from about0.5V and about 1V. Waveform 62 is the actual signal voltage output(e.g., voltage feedback) which is a 1 KHz sine waveform of the HVDC 28into a load of about 200 Ohms. In FIG. 5A, waveforms 60, 62 do not trackeach other, representing delay in discharging of the current. In FIG.5B, waveforms 60, 62 closer track each other due to faster dischargingcaused by the ADC 28.

FIGS. 5C and 5D shows a waveform 64 which represents gate drive of theswitching component 40. When the gate drive waveform 64 shows a rise itis representative of the switching component 40 being on therebyactivating a 5 Ohm load across the output. In FIG. 5C, the gate drivepulses on and off very rapidly, such as during the downward slopingportion of the waveforms 60, 62 the gate drive is on and off during theupward sloping portion of the waveforms 60, 62. FIG. 5D shows anexpanded view of the gate drive shows the rate of the pulsing, which isabout 230 KHz. This demonstrates that the ADC 28 is maintaining thesignal from falling below 0.5V.

FIGS. 5E and 5F show time differences between the waveforms 60, 62, 64.More specifically, FIG. 5E shows the time period between the pulse widthmodulator 28 turning off and the ADC 31 turning on is about 4 μs, whileFIG. 5F shows the time period between the pulse width modulator 28turning on and the ADC 31 turning off is also about 15 μs. Thisdemonstrates that the ADC 31 and the pulse width modulator 28 are notactivated at the same time, which reduces the risk of over-stressingcomponents of the generator 10.

The described embodiments of the present disclosure are intended to beillustrative rather than restrictive, and are not intended to representevery embodiment of the present disclosure. Various modifications andvariations can be made without departing from the spirit or scope of thedisclosure as set forth in the following claims both literally and inequivalents recognized in law.

What is claimed is:
 1. A circuit for discharging stored energy in anelectrosurgical generator, comprising: a pulse width modulator forcontrolling a high voltage power supply having an output; an errorsignal generating circuit configured for delivering an error signal as adifference between an output signal voltage and a feedback voltagegenerated by the high voltage power supply; a switchable load connectedin parallel with the output of the high voltage power supply; and aswitching circuit coupled to the error signal generating circuit, theswitching circuit configured to switch in the switchable load inparallel with the output of the high voltage power supply to dischargethe output in response to the error signal being below a firstpredetermined threshold.
 2. A circuit as in claim 1, wherein the firstpredetermined threshold is about 0.5V.
 3. A circuit as in claim 1,wherein the pulse width modulator turns off in response to the errorsignal being below a second predetermined threshold.
 4. A circuit as inclaim 3, wherein the second predetermined threshold is about 0.7V.
 5. Acircuit as in claim 1, further comprising at least one diode fordirecting current from the high voltage power source.
 6. A circuit as inclaim 1, wherein the switching circuit includes: a resistive element;and a switching component.
 7. A circuit as in claim 6, wherein theswitching component is a transistor selected from the group consistingof a field-effect transistor, a metal-oxide semiconductor field-effecttransistor, and an insulated gate bipolar transistor.
 8. Anelectrosurgical generator, comprising: a high voltage power source forgenerating direct current and having an output; a radio frequency outputstage for converting direct current into radio frequency energy; and acircuit including: a pulse width modulator for controlling the highvoltage power source; an error signal generating circuit configured fordelivering an error signal as a difference between an output signalvoltage and a feedback voltage generated by the high voltage powersource; a switchable load connected in parallel with the output of thehigh voltage power source; and a switching circuit coupled to the errorsignal generating circuit, the switching circuit configured to switch inthe switchable load in parallel with the output of the high voltagepower source to discharge the output in response to the error signalbeing below a first predetermined threshold.
 9. An electrosurgicalgenerator as in claim 8, wherein the first predetermined threshold isabout 0.5V.
 10. An electrosurgical generator as in claim 8, wherein thepulse width modulator turns off in response to the error signal beingbelow a second predetermined threshold.
 11. An electrosurgical generatoras in claim 10, wherein the second predetermined threshold is about0.7V.
 12. An electrosurgical generator as in claim 8, wherein thecircuit further includes at least one diode for directing current fromthe high voltage power source.
 13. An electrosurgical generator as inclaim 8, wherein the switching circuit includes: a resistive element;and a switching component.
 14. An electrosurgical generator as in claim13, wherein the switching component is a transistor selected from thegroup consisting of a field-effect transistor, a metal-oxidesemiconductor field-effect transistor, and an insulated gate bipolartransistor.
 15. An electrosurgical generator as in claim 13, wherein theresistive element has a resistance of about 5 Ohms.
 16. A method fordischarging energy stored in a circuit in an electrosurgical generator,comprising: deriving an error signal as a difference between an outputsetpoint voltage and a feedback voltage generated by a high voltagepower supply; comparing the error signal with a first predeterminedthreshold; transmitting the error signal to a switching circuitconfigured to switch in a switchable load; and switching in theswitchable load in parallel with an output of the high voltage powersupply to discharge the output in response to the error signal beingbelow the first predetermined threshold.
 17. A method as in claim 16,further comprising switching on a pulse width modulator in response tothe error signal being above a second predetermined threshold.
 18. Amethod as in claim 16, wherein the switching circuit includes: aresistive element; and a switching component.
 19. A method as in claim18, wherein the switching component is a transistor is selected from thegroup consisting of a field-effect transistor, a metal-oxidesemiconductor field-effect transistor, and an insulated gate bipolartransistor.
 20. A method as in claim 18, wherein the resistive elementhas a resistance of about 5 Ohms.